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Long before the service begins, Neapolitans dressed in their best clothes, file into their beautiful cathedral, past the TV cameras and the street vendors selling memorabilia. They are there to witness a miracle.

The cathedral is soon full and the aisles and side chapels are also quickly populated. The faithful are here to witness an event which has taken place for centuries, the liquefaction of the blood of their patron saint San Gennaro, a figure integral to the daily lives of people in this ancient port city.

The blood is held in a silver reliquary, a silver ornate holder, carrying a vial of what Neapolitans fervently believe to be the dried blood of Gennaro, who was beheaded for his faith by the Roman emperor Diocletian in the 4th Century.

The worshippers wait patiently, praying, singing and urging San Gennaro to reveal his presence through the liquefaction of his blood. They can only relax and celebrate when a white handkerchief is waved from the altar to tell them that the blood of San Gennaro has ‘miraculously’ turned from its normal dry state to liquid.

Locals believe that if the blood doesn’t liquefy then Naples will suffer terrible fortune, so when the white handkerchief is waved, the celebrations are due partly to relief.

In the first of two part series for Heart and Soul, Mark Dowd travels to Naples in southern Italy to witness for himself the blood miracle and to ask why miracles - these mystical, unexplainable supernatural events - are so vital to Christians and why these unexplainable events which seemingly defy science are signs for worshippers that the heavenly God is a tangible presence on Earth.

In the second part, Mark will ask why miracles are so important in sainthood, as the Catholic church prepares to beatify Popes John Paul II and John XXIII.

(Photo: Statue of San Januarius, the Saint protector of Naples. Credit: Gabriel Bouys/AFP/Getty Images)